Electrochemical fuel cells convert chemical energy derived from a fuel directly into electrical energy by oxidizing the fuel in the cell. Power plants that generate electrical energy from electrochemical fuel cells are of particular interest to utilities because they can provide incremental, dispersed electric power, thus overcoming some of the difficulties associated with conventional nuclear, coal or hydrocarbon fuel power plants, such as access to high voltage transmission lines, distribution to urban power stations, and the substantial financial commitments typically associated with conventional power plants. In addition, electrochemical fuel cell systems are capable of operating at greater than 40% electrical efficiency, and thus produce electrical energy more economically than conventional power plants, while substantially avoiding the so-called "green house" effect. Electrochemical fuel cell power plants are also relatively quiet, and produce minimal emissions. Thus, the disadvantages of environmental pollution, power plant location and regulatory approval are reduced in the case of fuel cell electric power generation systems.
At the present time, fuel cell power plants using liquid phosphoric acid as the electrolyte in the fuel cells are nearest to being commercialized. These power plants include a fuel processing subsystem, a fuel cell electric power generation subsystem, an air supply subsystem, a water cooling/recovery subsystem, a power conversion subsystem, and a control subsystem. The fuel processing subsystem generally incorporates steam reforming of the hydrocarbon fuel to produce the hydrogen-rich gas for the fuel cells. Because the phosphoric acid fuel cells operate at temperatures of about 400.degree. F., the cells are typically cooled by boiling water in coolers and the steam produced is then supplied to the fuel processing subsystem. The use of fuel cell stack waste heat to produce the steam is an efficiency improvement for the fuel processing subsystem in phosphoric acid fuel cell power plant systems because extra fuel does not have to be burned in the fuel processing subsystem to produce the steam. This improvement in fuel processor efficiency means that the phosphoric acid fuel cell electric power generation subsystem can be lower in efficiency to achieve the desired overall power system efficiency. This permits the phosphoric acid fuel cells to operate at lower voltage and hence higher power density.
Solid polymer fuel cells typically operate at about 180.degree. F. This temperature is too low to produce steam for the fuel processing subsystem. A steam generator (vaporizer or evaporator) is therefore incorporated in the fuel processing subsystem of solid polymer fuel cell systems to produce steam. The vaporizer derives energy to convert water to steam from the reformer burner exhaust stream. The energy in the reformer burner exhaust stream to drive the vaporizer is produced by burning extra fuel in the reformer burner. The burning of extra fuel in the reformer burner effectively lowers the efficiency of the fuel processing subsystem in solid polymer fuel cell power plant systems. Thus, in order to compete effectively with phosphoric acid fuel cell power plant systems, solid polymer fuel cell power plant systems must provide increased efficiency in other subsystems to achieve the same overall system electrical efficiency. In the solid polymer fuel cell power plant system disclosed herein, the increase efficiency is provided in the electric power generation subsystem, wherein the fuel cells are operated at a higher voltage than that permitted in the fuel cells of phosphoric acid power plant systems. Because, however, the solid polymer fuel cells in the electric power generation system disclosed herein have extremely favorable performance characteristics, the power density of the solid polymer fuel cells at the higher operating voltages is greater than that of phosphoric acid fuel cells at the lower operating voltage of phosphoric acid power plant systems.
The solid polymer fuel cell electric power generation system disclosed herein comprises the following subsystems: (1) an air pressurization subsystem, (2) a fuel processing subsystem, (3) an electric power generation subsystem, (4) a water recovery subsystem, (5) a power conversion subsystem, and (6) a control subsystem. Atmospheric air is introduced into the air pressurization subsystem, which, in a preferred embodiment, comprises a two-stage turbocompressor. The air stream is cooled between compression stages for efficient compression, pressurized, then humidified using water from the water recovery subsystem before being introduced as the oxidant stream into the electric power generation subsystem.
Natural gas (methane) or liquid hydrocarbon fuel is the raw fuel source introduced into the fuel processing subsystem. The fuel stream is heated (and vaporized if supplied as a liquid) and converted in a steam reformation process, further including shift reactors and a selective oxidizer, into a hydrogen-rich reformate fuel stream. The reformate fuel stream is introduced as the fuel stream to the fuel cell based electric power generation subsystem. The particular components of the fuel processing subsystem are largely dependent upon the type of raw fuel used. In addition to methane (natural gas), the raw fuel source may be methanol, fossil fuels, garbage off-gas, or other hydrogen-containing substances. In a preferred embodiment utilizing natural gas as the fuel, the fuel processing subsystem comprises an electric driven compressor to pressurize the fuel, a preoxidizer catalyst bed to consume oxygen from the fuel, a hydrodesulfurizer to remove sulphur from the fuel, a vaporizer to impart water from the water recovery subsystem to the fuel stream, a regenerator heat exchanger to heat the fuel stream, a steam reformer to produce a hydrogen-rich reformate fuel stream, a regenerator heat exchanger and shift convertor precooler heat exchanger to cool the reformate fuel stream, a shift reactor for converting carbon monoxide in the reformate fuel stream to carbon dioxide and hydrogen, a selective oxidizer precooler heat exchanger, a selective oxidizer to further convert carbon monoxide in the reformate fuel stream to carbon dioxide, an anode precooler, and a water separator to remove water from the reformed fuel stream.
The electric power generation subsystem employs an electrochemical fuel cell stack to oxidize the reformate fuel stream and generate electricity, heat and product water. In a preferred embodiment, the fuel cells comprise a membrane electrode assembly comprising a solid polymer membrane such as DuPont's Nafion brand ion exchange membrane or Dow's experimental ion exchange membrane XUS 13204.10. The ion exchange membrane is disposed between two electrodes formed of porous, electrically conductive sheet material, preferably carbon fiber paper, each having a layer of platinum catalyst disposed at the membrane/electrode interface to render the electrodes electrochemically active. The membrane electrode assembly is typically disposed between graphite flow field plates engraved with flow passages which direct the fuel stream and oxidant stream, respectively, to the adjacent anode electrode and cathode electrode.
At each anode, the hydrogen-rich reformate fuel stream permeates the porous electrode structure of the anode and reacts at the catalyst layer to form cations (hydrogen ions or protons), which migrate through the membrane to the cathode. In addition to facilitating the migration of the cations, the membrane isolates the hydrogen fuel stream from the oxidant stream. At each cathode, the oxygen-containing oxidant stream reacts at the catalyst layer at the cathode/membrane interface to form anions. The anions formed at each cathode react with the cations to complete the electrochemical reaction and form water as a reaction product. The electrodes are electrically coupled to provide a path for conducting electrons between the anode and the cathode through an external load.
The water produced in the electric power generation subsystem is recycled in the water recovery subsystem. Because the solid polymer fuel cell operates at temperatures lower than phosphoric acid electrolyte fuel cells, the water produced in the electric power generation subsystem is readily separated from the fuel cell exhaust stream, without the need for large, expensive condensing equipment. In a preferred embodiment of the present fuel cell electric power generation system, the recycled water is employed (1) as the vaporized water fed to the fuel processing subsystem, (2) as the water for humidification in the air pressurization subsystem, and (3) as the coolant for the fuel cell stack. Another embodiment has a closed coolant loop for the fuel cell stack, with the recycled water employed as the source of water for (1) and (2) above, and a separate stream provides coolant to the fuel cell stack (item (3) above).
One of the advantages of the present system is the location of the humidification function outside the fuel cell stack, thus enabling the fuel cell to be smaller and less costly to manufacture. Because the ability of gases to absorb water vapor varies significantly with temperature, especially at low pressure, it was previously thought that humidification of the fuel and the oxidant should take place within the fuel cell. Humidification within the fuel cell ensures that the humidification takes place at or as close as possible to the operating temperature and pressure of the fuel cell. It has now been found, however, that the performance and lifetime of the system is improved if humidification takes place outside of the fuel cell, where the temperature of the humidification process can be controlled to produce reactant streams having the humidification level most desirable for a given set of fuel cell operating conditions.
In the present system, unreacted hydrogen and oxygen which exits the electric power generation subsystem is exhausted and reused in the fuel processing subsystem and the air pressurization subsystem. Excess water is separated from the reactant exhaust streams, and the exhaust streams are then directed to the burner portion of the fuel processing subsystem, where the energy from the exhaust streams is used to power the reformer. The waste energy from the reformer is in turn used to power the two stage turbocompressor in the air pressurization subsystem and to provide heat for making steam for the fuel processing subsystem.
The electricity produced by the electric power generation subsystem is unregulated DC and is converted into regulated, utility grade AC power by the power conversion subsystem. The power conversion subsystem comprises an inverter for converting the electricity from DC to AC and a battery peaking unit connected to the inverter which supports the fuel cell stack voltage during transient increased demand periods in rapid load-following applications.
It is therefore an object of the present invention to provide an efficient, high power density solid polymer fuel cell electric power generation system that produces utility grade electrical power from gaseous or liquid hydrocarbon fuels.
Another object of the invention is to provide a solid polymer fuel cell electric power generation system that makes productive use of waste energy normally exhausted from conventional systems.
Yet another object of the invention is to provide a solid polymer fuel cell power generation system that competes directly with phosphoric acid fuel cell electric power generation systems by employing a solid polymer fuel cell stack which operates at a higher efficiency than corresponding phosphoric acid fuel cell structures.
A further object of the invention is to provide a solid polymer fuel cell electric power generation system that recycles water generated as a reaction product by the system.
A still further object of the invention is to provide a solid polymer fuel cell electric power generation system that can perform the reactant humidification function outside of the fuel cell stack.